This paper examines an asymmetric spatiotemporal connection and climatic impact between the winter atmospheric blocking activity in the Euro-Atlantic sector and the life cycle of the North Atlantic Oscillation (NAO) during the period 1950-2012. Results show that, for positive NAO (NAO+) events, the instantaneous blocking (IB) frequency exhibits an enhancement along the southwest-northeast (SW-NE) direction from the eastern Atlantic to northeastern Europe (SW-NE pattern, hereafter), which is particularly evident during the NAO+ decaying stage. By contrast, for negative NAO (NAO-) events, the IB frequency exhibits a spatially asymmetric southeast-northwest (SE-NW) distribution from central Europe to the North Atlantic and Greenland (SE-NW pattern, hereafter). Moreover, for NAO- (NAO+) events, the most marked decrease (increase) in the surface air temperature (SAT) in winter over northern Europe is in the decaying stage. For NAO+ events, the dominant positive temperature and precipitation anomalies exhibit the SW-NE-oriented distribution from western to northeastern Europe, which is parallel to the NAO+-related blocking frequency distribution. For NAO- events, the dominant negative temperature anomaly is in northern and central Europe, whereas the dominant positive precipitation anomaly is distributed over southern Europe along the SW-NE direction. In addition, the downward infrared radiation controlled by the NAO's circulation plays a crucial role in the SAT anomaly distribution. It is further shown that the NAO's phase can act as an asymmetric impact on the European climate through producing this asymmetric spatiotemporal connection with the Euro-Atlantic IB frequency.

Fig. 1 (a) Geographical distribution of the climatological winter IB frequency during the winter period (NDJFM) from 1950 to 2012. Shading is representative of the percentage of IB days with respect to the total days of a winter. The lines of latitude are plotted at 5° intervals starting at 20°N. Units: %. (b, c) Composite normalized daily NAO indices in winter for (b) in-situ NAO+ and (c) in-situ NAO- events during winter 1950-2012. B1, B2, A1 and A2 represent four sub-periods of the NAO life cycle. B (A) means before (after) the peak day lag(0).

Fig. 2 Geographical distribution of IB frequencies in winter averaged for four sub-periods of NAO events: (a) NAO+ events; (b) NAO- events. Shading is representative of the percentage of IB days with respect to total NAO days within each stage. The lines of latitude are plotted at 5° intervals starting at 20°N. Units: %.

Fig. 6 Sketch map of the relationship between the blocking distribution associated with (a) NAO+ and (b) NAO- events and the temperature and precipitation anomalies over Europe. The red/blue shading in (a)/(b) represents the positive/negative temperature anomaly region; the positive precipitation anomaly is marked by green shading.

Classification

NAO+-IB+

NAO+-IB-

NAO--IB+

NAO--IB-

Number of cases

51

46

45

54

SAT n

70%

30%

69%

31%

SAT s

78%

22%

93%

7%

Pre n

86%

14%

82%

18%

Pre s

75%

25%

66%

34%

Table 2. Contribution of the IB frequency to SAT and precipitation (Pre) during NAO events. An NAO+-IB+ (NAO+-IB-) event represents an NAO+ event with higher (lower) IB frequency over southern Europe than the mean value of all NAO+ events. An NAO--IB+ (NAO--IB-) event represents an NAO- event with higher (lower) IB frequency over northern Europe than the mean value of all NAO- events. SAT n (Pre n) and SAT s (Pre s) mean the SAT (Pre) over northern and southern Europe, respectively.

Fig. 8 Trajectory tracking of the composite daily geopotential height dipole anomalies in winter during the life cycles of (a) NAO+ events and (b) NAO- events. The red (blue) markers indicate the maximum (minimum) anomaly position for a positive (negative) anomaly center for the NAO dipole.

Fig. 9 Spatial distribution and latitudinal profile of time-mean 300-hPa zonal winds from lag(-10) to lag(+10) averaged for (a) NAO+ events and (b) NAO- events. Units: m s-1. Dark (light) shading denotes regions of positive (negative) anomalies above the 95% confidence level for a two-sided Student’s t-test. Contours are drawn at intervals of 2 m s-1. The solid (dashed) line in the right-hand part of each panel represents the zonal wind profile averaged over the Atlantic basin (Europe) for the region 60°-10°W (10°W-60°E).

Sung M. K.,G. H. Lim, J. S. Kug, and S. I. An, 2011: A linkage between the North Atlantic Oscillation and its downstream development due to the existence of a blocking ridge. J. Geophys. Res.,116, D11107.

Yao Y.,D. H. Luo, 2014: Relationship between zonal position of the North Atlantic Oscillation and Euro-Atlantic blocking events and its possible effect on the weather over Europe. Science China Earth Sciences,57, 2628-2636.

This paper examines an asymmetric spatiotemporal connection and climatic impact between the winter atmospheric blocking activity in the Euro-Atlantic sector and the life cycle of the North Atlantic Oscillation (NAO) during the period 1950-2012. Results show that, for positive NAO (NAO+) events, the instantaneous blocking (IB) frequency exhibits an enhancement along the southwest-northeast (SW-NE) direction from the eastern Atlantic to northeastern Europe (SW-NE pattern, hereafter), which is particularly evident during the NAO+ decaying stage. By contrast, for negative NAO (NAO-) events, the IB frequency exhibits a spatially asymmetric southeast-northwest (SE-NW) distribution from central Europe to the North Atlantic and Greenland (SE-NW pattern, hereafter). Moreover, for NAO- (NAO+) events, the most marked decrease (increase) in the surface air temperature (SAT) in winter over northern Europe is in the decaying stage. For NAO+ events, the dominant positive temperature and precipitation anomalies exhibit the SW-NE-oriented distribution from western to northeastern Europe, which is parallel to the NAO+-related blocking frequency distribution. For NAO- events, the dominant negative temperature anomaly is in northern and central Europe, whereas the dominant positive precipitation anomaly is distributed over southern Europe along the SW-NE direction. In addition, the downward infrared radiation controlled by the NAO's circulation plays a crucial role in the SAT anomaly distribution. It is further shown that the NAO's phase can act as an asymmetric impact on the European climate through producing this asymmetric spatiotemporal connection with the Euro-Atlantic IB frequency.